There is known an internal combustion engine in which blowby gas that leaks from a combustion chamber is introduced into an intake passage (see Japanese Laid-Open Patent Publication Nos. 2010-209843 and 2010-90870).
The internal combustion engines may have a situation in which fuel that adheres to a wall of the combustion chamber under cold engine conditions is mixed with oil and is vaporized under engine warmed-up conditions, whereby a large amount of fuel is included in the blowby gas. Specifically, in an internal combustion engine capable of using an alcohol mixed fuel, since the alcohol component of fuel included in oil is abruptly vaporized at the boiling point (approximately 78° C. for ethanol, and approximately 65° C. for methanol), a large amount of fuel is likely to be included in the blowby gas. Since the fuel component is included in the blowby gas in this way, the blowby gas may include fuel as much as an amount that makes it impossible to control the air/fuel ratio within an appropriate range by a feedback control that corrects the amount of fuel injection.
In this case, the air/fuel ratio may be adjusted by reducing the flow rate of the blowby gas by a valve that is provided in a passage through which the blowby gas is introduced to an intake passage from a crankcase, for example. In the introduction of the blowby gas into the intake passage from the crankcase, fresh air as much as the blowby gas introduced into the intake passage can be introduced into the crankcase from the intake passage through another passage.
However, in the case where a large amount of fuel is included in the blowby gas, the pressure in the crankcase is raised due to the pressure of the vaporized fuel. Thus, in this case, even when the flow rate of the blowby gas is reduced by the above valve, blowby gas may be introduced into the intake passage from the crankcase through the passage that is originally used to introduce fresh air into the crankcase from the intake passage. As a result, even when the flow rate of the blowby gas is reduced by the above valve, there is a possibility that the air/fuel ratio is not adjusted.